Voltmeter
   The analogue voltmeter is a galvanometer movement with a multiplier resistor in series with it. The potential drop across the galvanometer itself is small, and the value of the multiplier is adjusted to allow most of the potential drop to measured to take place across it.

   The Weston Direct Reading Laboratory Standard Ammeter and Millivoltmeter at the left below was bought by Denison University in October 1905 at a cost of $100.00 Next to it is the Weston Multiplier used with it, that cost $27.00.
 

   The two Weston Millivoltmeters below are essentially the basic D'Arsonval meter movement without any multipliers. They are good for showing induced EMFs in the lecture room. They remind me of small, bowlegged men, and I always expect them to walk away.
                                Denison University                                                                       Kenyon College
   The voltmeter at the right is from DePauw University, and the mounting board shows the workmanship of Prof. Joseph Naylor

   The instrument itself is by Queen of Boston. Examination of its face shows that it is a magnetic vane voltmeter, with a resistance of 58 Ohms at 15°C. 

   The 1887 catalogue of James W. Queen of Philadelphia has a complete discussion of this voltmeter, made by the Paris firm of Deprez-Carpentier: "[It] contains a needle of soft iron, movable on an axis in the interior of a coil of wire or ribbon connected to the two binding posts on the side. Two magnets, in the form of a C, placed opposite each other, enclose the coil and needle between the coils. The axis of the soft iron needle is also fastened to the aluminum indicating pointer, and the whole system is so balanced that the instrument can be employed in either a horizontal or vertical position, placed on a table, or hung against a wall."

   This is the face plate of a Deprez-Carpentier voltmeter in the collection of historical instruments at Washington and Lee University in Lexington, Virginia. 

   The voltmeter had a coil made up of many turns of fine wire. When this coil was made of a few turns of heavy wire, the instrument was an ammeter. 

   The voltmeters cost upwards of $32.00, depending on whether multiplier coils were included, and the ammeters cost upwards of $24.00, with the higher prices including shunts for higher current ranges. 

   The iron-vane instruments below are in the Garland Collection of Classical Physics Apparatus at Vanderbilt University.
 


   The Kilovoltmeter at the right is in the Kenyon College Collection.

   It was made by the Hickok Electrical Instrument Co. of Cleveland, O, and bears the label (in front) of the Kelley Koett Mfg. Co., Inc. of Covington, Kentucky (across the Ohio River from Cincinnati). It is serial number 30007 and is marked off in units of 250 V. 

   The Volt-Ammeter at the left has a movement and case by the L.E. Knott Apparatus Co. of Boston. 

   However, it appears to have had some additions to turn it into a dual-use instrument. Presumably there is one terminal (probably negative) in common. One of the other terminals have a large multiplier resistance to convert the basic galvanometer into a voltmeter, while the third one leads to the basic movement and a shunting resistor to form the ammeter. The instrument has been hand-calibrated. 

   It is retired from the State University of New York at Fredonia. 

   The 1929 central scientific Company catalogue lists a combination volt-ammeter with ranges of 0-10 volts and 0-10 amperes in 0.1 unit divisions. The model in the standard case was $20.00, but for $25.00 you could get the instrument at the right with a glass dome covering the works, thus making them visible to students. It is in the Greenslade Collection.

   The view is somewhat obscured by inhomogenieties in the glass, but the pole pieces and core ["carefully machined from annealed stock which has been carefully selected after many experiments as the most suitable for producing a uniform magnetic field in the air gap throughout the range of deflection"]. 

   The positive terminal is common, and the other two terminals are for connection as a voltmeter or as an ammeter. Depressing the white button actuates the instrument.

   Soon after putting the demonstration meter above into this page, I looked on a shelf in the electronics lab at Kenyon during an afternoon experiment, and realized that the College owned the twin to the demo meter. The mother-of-pearl button was the identifying point. 

   In the 1929 Central Scientific catalogue this device is shown with a metal base, leading me to conclude that the example at the left is somewhat earlier. 

   The direct-current voltmeter at the right must have a very large permanent magnet, as it is remarkably heavy. One of Edward Weston's innovations for moving-coil electrical meters is a permanent magnet that has been aged to reduce its tendency to change its magnetic field with time. 

   This instrument is dual range: 0 to 5 V and 0 to 50 V. The information cast into the face-plate tells us that it was made by the Weston Electrical Instrument Company of Newark, New Jersey, U.S.A.; its numerous patent dates run from 1888 to 1901. An interesting feature is the auxiliary pointer with the circular end that can be set with the shiny knob on the face-plate. This lines up with the hole in the pointer.

   This instrument is in the Greenslade Collection.

 
   This picture of a volt/ammeter is almost life size; the instrument is just a little larger than a pocket watch. The negative contact is the sharpened pin at the bottom. The red electrode is for the voltmeter with its 0 to 10 V scale, while the green electrode drives the ammeter, which reads up to 25 amperes.

   The instrument is in the Greenslade Collection
 



   The handsome voltmeter at the right was made by the American Instrument Company of Newark, New Jersey. It has patent dates from 1906 and 1907.

   The instrument is at Westminster College in western Pennsylvania. Its twin is shown on the ammeter page.





   The Jewell Standard Voltmeter at the left almost qualifies as decorative art! It was made by the Jewell Electrical Instrument Company of Chicago, and has an August 29, 1899 patent date.

   The voltmeter is in the collection of historical instruments at Case Western Reserve Unviersity in Cleveland, Ohio.

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